Recently, the demand for finding alternative energies has increased and thus there is a growing interest in the technology of recovering latent heat from low-temperature exhaust gas or coolant and of recycling it.
In this regard, a stirling engine is used to convert low- or medium-temperature heat energy into shaft power (high quality energy). A stirling engine is advantageous in that it has a very simple structure and is operated in an easy way because all the elements thereof involved in the Rankine cycle are integrated into one engine and gas such as air is used as a working fluid.
In addition, a stirling engine has the highest heat efficiency among systems using the Rankine cycle. Therefore, when converting low- or medium-temperature heat energy into work, the use of the stirling engine is advantageous in terms of its relatively simple structure and high energy conversion efficiency compared to conventional Rankin-cycle systems.
As illustrated in FIG. 1, recently a micro-combined heat and power (micro-CHP) generator that is a power generation system capable of generating electricity and heat using a stirling engine has been domestically used. Such a micro-CHP is a kind of a boiler system for domestic use and it includes a stirling engine 110 and an auxiliary boiler 200, in which the stirling engine 110 generates electricity and the auxiliary boiler 200 generates hot water for heating.
A high-temperature heat medium, supplied via a sensible heat exchanger 210 and a latent heat exchanger 220 included in the auxiliary boiler 200, undergoes heat exchange with low-temperature water in a hot water tank 300 to produce hot water and thus becomes a low-temperature heat medium by losing heat through the heat exchange. The hot water is stored in the hot water tank and then consumed by a user. The low-temperature heat medium produced through the heat exchange performed in the hot water tank 300 returns to the stirling engine through a heat medium return line 130 to cool down the stirling engine 110. The low-temperature heat medium sequentially passes again the latent heat exchanger 220 and the sensible heat exchanger 210 to become a high-temperature heat medium by gaining heat from the sensible and latent heat exchangers after passing through the stirling engine 110. In this way, the heat medium circulates and repeatedly undergoes this heat exchange cycle.
In the stirling engine 110, when an engine head (not shown) is heated, a working fluid (for example, hydrogen gas or helium gas) in the engine head expands and contracts due to a temperature difference and thus generates alternating current. As the difference in temperature between a high temperature portion that is the engine head heated by an engine burner 120 and a low temperature portion that is a portion to which the heat medium returns is increased, an amount of electricity generated is increased.
The hot water tank 300 is provided with a temperature sensor 310 to turn on the micro-CHP when the temperature of the hot water in the hot water tank is lowered to below a predetermined temperature, so that the hot water tank 300 can normally store hot water having a temperature in the range of 50° C. to 60° C. In addition, when a certain amount of hot water is consumed, water is externally supplied to the hot water tank through a direct water line by an amount equal to the consumed amount, so the water in the hot water tank is replenished. Thus, the water in the hot water tank can be maintained at a predetermined level.
However, when the hot water in the hot water tank is consumed by a user, the stirling engine is operated and thus the water in the hot water tank 300 is heated up to a temperature in the range of 55 to 65° C. At this point, the temperature of the heat medium that returns to the stirling engine through the heat medium return line 130 may normally reach a temperature in the range of 60 to 70°.
In this case, since the temperature of the low temperature portion of the stirling engine 110 is increased by the heat medium that returns through the heat medium return line 130, there is a problem that an amount of electricity that is produced is decreased.
Furthermore, the operation of the stirling engine 110 is partially or entirely restrained when the temperature of the returning heat medium is 60° C. or higher so that the stirling engine can be protected from the thermal shock caused by the high-temperature heat medium that returns to the stirling engine. Thus, the stirling engine 110 repeatedly stops and resumes operating in accordance with the temperature of the low temperature portion of the stirling engine 110, which is likely to result in durability deterioration. Therefore, it is difficult to expect a trouble-free operation of the stirling engine.